Commercial air separation practice involves the distillation of air at cryogenic temperature levels. The distillation of the air process stream occurs at temperature levels of about 80.degree.-100.degree. K. compared to ambient conditions of about 300.degree. K. The separation process includes the use of appropriate heat exchangers to cool the feed air from ambient to distillation temperatures and recover the refrigeration from the separated cold return streams. The air desuperheating step is typically performed at relatively low temperature differences in order to avoid large energy expenditures. The heat exchangers typically utilized are of a plate and fin construction which are fabricated by stacking alternate layers of aluminum parting sheets and corrugated fin stock and brazing the entire structure to form the required mechanical rigidity. The heat exchangers commonly used for the air desuperheating step have an additional function that involves the removal of the usual atmospheric air contaminants such as water and carbon dioxide. Such process arrangement utilizes what is commonly referred to as the reversing heat exchanger (RHX) arrangement in which air contaminants which deposit on a given heat exchanger passage during a portion of the operation are removed by subsequent removal of that passage from air service and sweeping that passage with waste nitrogen to remove the contaminant. The reversing heat exchanger arrangement utilizes the difference in pressure between the air feed stream and returning waste nitrogen stream to remove the contaminant from the air stream and prevent clogging of downstream processing equipment.
Since the air separation process typically utilizes ambient air compressed to an increased pressure of about 90-100 psia, it is common for the feed air stream to be substantially saturated with water vapor at the entrance to the reversing heat exchanger section. As the air proceeds to cool, the water vapor is condensed and forms a liquid film on the heat exchanger surfaces. As the air continues to cool a point is reached where the temperature corresponds to the freezing point of water and the water vapor then continues to be removed but is deposited directly as a snow or ice film on the surfaces. At still lower temperature levels, the carbon dioxide contaminant begins to plate out and is again removed as a snow or solid film on the heat exchanger surface. During the subsequent cleaning stroke of the reversing heat exchanger sequence, the waste nitrogen serves to revaporize the carbon dioxide and water and remove it from the heat exchanger. This sequential switching of the heat exchanger passages maintains the surfaces in a relatively clean and functional manner and prevents the introduction of the contaminants into the colder regions of the process equipment where the solid materials would serve to impede the operation of the equipment. Since ambient air available in typical industrial environments often contains trace quantities of corrosive elements such as sulfur compounds, chlorine compounds, or other compounds, these components are carried along with the air into the reversing heat exchangers. The water film that generally coats the upper regions of the reversing heat exchangers tends to concentrate these contaminants and form a mildly corrosive solution that attacks the aluminum material of the heat exchanger. Over a substantial operating period, such corrosive attack of the aluminum heat exchanger may eventually cause mechanical failure of the heat exchanger and thereby require replacement of the unit.
Since the corrosion of the aluminum reversing heat exchanger due to atmospheric contaminants concentrated in the water condensate has been a continual problem, and leads to additional expense associated with the replacement of the heat exchanger units, many attempts have been made in the past to solve the corrosion problem. The brazed aluminum heat exchangers utilized for the air desuperheaters are of the plate and fin design. Such units involve specialized and costly furnace brazing operations following the stacking of the parting sheets and corrugated fins that make up the heat exchanger core. The heat exchanger parting sheets that form the passage low channels are relatively thin aluminum stock ranging from 16 to 64 mils in thickness and corrosion protection of these elements are extremely important to heat exchanger life. Direct corrosion protection to the parting sheets and/or corrugated fins would complicate the brazing operations and substantially increase the fabrication cost of the heat exchanger. Therefore only indirect corrosion protection to the heat exchanger, particularly the core, is practical. Previous attempts to provide indirect corrosion protection by the brazed aluminum heat exchanger core have been associated with procedures and techniques involving treatment of the upstream air supplied to the heat exchanger. These attempts to solve the problem have included the use of galvanized air piping upstream of the heat exchanger, use of zinc demister pads in air inlet piping, and proposals to insert sacrificial zinc alloy anodic bar members in the air stream to afford cathodic protection to the downstream aluminum heat exchanger core. None of the above methods have been entirely satisfactory for various reasons.
The use of zinc coated or galvanized piping has served to protect the air piping but offers only small improvements for corrosion reduction of the heat exchanger core. The air piping itself is constructed of relatively heavy stock material and thereby corrosion of that member is not a problem. From a galvanic action standpoint, the zinc attached to the piping has little or no impact on corrosion inhibition in the heat exchanger core. The zinc demister pad technique was partially successful in that it would serve to remove suspended and entrained water condensate (with its dissolved corrosives) and prevent its detrimental action on the downstream heat exchanger. However, it has little effect on water content associated with the saturated air which would of course deposit on the heat exchanger as the air was cooled. Still additionally, that technique was a problem because with continued service the screen members associated with the pad would corrode and degenerate and eventually break off and carryover as undesirable debris into the inlet of the heat exchanger. Of course, when this breakage of the screen took place, any subsequent protection for the heat exchanger was not available. The anodic member suspended in the air inlet stream was not entirely satisfactory for at least two reasons. First, it is difficult to expose all or substantially all of the air stream to the associated anodic action by the insertion of one or more members into the air flow. Second, such insertion of bar members into the air flow imposed an undesirable air flow restriction in the inlet piping. Trial field tests indicated that with eventual corrosion and degradation of the anodic members there was subsequent breakage and carryover of such members into the air heat exchanger. This again served to introduce undesirable debris into the inlet of the heat exchanger.
The continuing problem associated with the corrosion of reversing heat exchangers and undesirable features associated with the various attempts to solve the problem set the stage for the improved corrosion protection technique associated with this invention. It was discovered that indirect corrosion protection of the aluminum heat exchange core could be achieved by employing a manifold arrangement of headers in which predetermined headers are coated to function as sacrificial anodes in addition to directing fluid flow into preselected fluid channels. The coated headers indirectly provide corrosion protection to the aluminum core of the heat exchanger. Only the air and waste nitrogen headers are coated using a composition of zinc and aluminum preferably a zinc aluminum alloy. Such coating is preferably applied by any conventional thermal spray process. Additionally, the alloy coating should be strategically located directly upstream of the aluminum heat exchanger core at the warm end thereof. The cathodic corrosion protection established by the applied coating is due to the electrical action between the applied coating and the aluminum core with the circuit required for such electrical action formed by the water condensate film present in the heat exchanger. Test work has shown that the water condensate film associated with reversing heat exchanger operation can supply sufficient electrical conductivity to afford cathodic protection to the uncoated heat exchanger core downstream of the coated headers. It is common practice to use a sacrificial anode only under immersion conditions with a ready electrolytic conductive path. The dissolution of the sacrificial anode alloy coating serves to inhibit the corrosive activity otherwise due to the solution of corrosive agents normally present in the ambient feed air.